1. Field of the Invention
The present invention relates generally to a process for converting Fischer-Tropsch light olefin-containing fractions into alkylates. More particularly, this invention relates to a process for obtaining high octane alkylates by reacting Fischer-Tropsch light olefins containing low levels of oxygenates with isoparaffins.
2. Description of Related Art
Because of their high octane numbers and low vapor pressures, alkylates have been used for many years as blending components in motor gasolines. The alkylation process involves the reaction of light olefins such as propylene and butylene with isoparaffins such as isobutane and isopentane in the presence of an acid catalyst such as H2SO4 and HF to form highly branched, isoparaffinic products known as alkylates. Typical sources for light olefins include catalytic crackers, cokers and vis-breakers.
The Fischer-Tropsch process also produces light olefins. The process involves reacting synthesis gas composed mainly of CO and H2 in the presence of a suitable catalyst to form a variety of predominantly linear hydrocarbonaceous solid, liquid and gaseous products. Some of these products can be refined using known procedures such as hydrotreating, hydrocracking and hydroisomerization to yield moderately branched, isoparaffin-rich middle distillate fuels such as diesel and jet fuels. A gaseous phase produced in a Fischer-Tropsch synthesis, upon condensation and subsequent distillation, produces a light olefin product composed primarily of C3-C4 olefins. However, this product is considered to be poorly suited for use as a feedstock for conversion to alkylates for a number of reasons.
Products of Fischer-Tropsch syntheses normally contain relatively high levels of oxygenates, frequently in amounts above 4000 ppm oxygenate. Oxygenates can react to form water which dilutes the acid catalysts conventionally used in alkylation. Accordingly, light olefin feedstocks for alkylation should contain no more than 4000 ppm oxygenate, preferably much less. Also, light olefin streams from Fischer-Tropsch processes contain relatively low levels of isobutane. As such, they do not constitute a good source for isobutane. In the alkylation process, isobutane feedstocks should contain at least 30% by weight, preferably up to 75% by weight isobutane. Also, Fischer-Tropsch light olefin streams are composed predominantly of 1-butene. Feeds containing 1-butene normally yield alkylates with lower octane numbers than 2-butene. Desirably, the ratio of 2-butene to total butenes in the feed for alkylation should be at least 0.1 and most preferably at least 0.5.
Hydroisomerization processes have been used to produce moderately branched iosparaffins in the distillate fuel boiling range. However, they cannot be used to produce high-octane, highly branched isoparaffins in the gasoline boiling range. If hydroisomerization processes are run in a severe mode in an attempt to create highly branched products instead of moderately branched products, the feedstock cracks to form excessive amounts of undesirable light gases. Accordingly, the only practical method to manufacture high octane, highly branched, gasoline boiling range isoparaffins is by alkylation.
Consideration has been given to converting Fischer-Tropsch C3-C4 olefin streams into a feedstock suitable for alkylation by total hydrogenation or by using known treatments to lower the oxygenate content to an acceptable level. However, these approaches would necessitate a separate step to isomerize butane while hydrogenation would saturate the olefins in the C4 stream. Also, the oxygenates removed from the C3-C4 stream would not be converted into alkylates. What is needed is an economical process for converting a C3-C4 light olefin fraction from a Fischer-Tropsch process into a highly branched isoparaffin mixture suitable as a blending component to prepare high octane gasolines.
Alkylation using light olefins obtained via a Fischer-Tropsch synthesis are described in U.S. Pat. Nos. 4,279,830; 4,046,830; and 4,049,741. These patents do not disclose using a C3-C4 olefin-containing feed which has been processed to reduce oxygenate levels to below 4000 ppm oxygenate.
It is an object of the invention to provide a process for economically preparing high octane alkylates from a Fischer-Tropsch C3-C4 light olefin stream having reduced oxygenate levels.
It is another object of the invention to utilize a Fischer-Tropsch C3-C4 olefin fraction with reduced oxygenate levels and an isobutane stream obtained by hydrocracking a Fischer-Tropsch 300xc2x0 F.+ product to produce a high octane alkylate.
These and other objects of the present invention will become apparent to the skilled artisan upon a review of the following description, the claims appended hereto and the Figures of the drawings.
The objectives of the invention are attained by a process which includes the following steps:
(a) recovering a light C3-C4 olefin product stream from a Fischer-Tropsch reactor;
(b) processing the C3-C4 olefin stream to reduce the level of oxygenates to less than 4000 ppm oxygen;
(c) hydrocracking a C5+ product from a Fischer-Tropsch synthesis to generate an isobutane-containing stream;
(d) blending the product stream from step (b) with the product stream from step (c);
(e) reacting the blend of step (d) in the presence of an alkylation catalyst; and
(f) recovering a highly branched, isoparaffinic alkylate having a research octane number greater than 80.
Key features of the invention include processing a Fischer-Tropsch C3-C4 olefin stream to be used in an alkylation reaction so that it contains no more than about 4000 ppm oxygenate and preferably less than 1000 ppm, and processing a C5+ Fischer-Tropsch product to produce a feed for alkylation containing at least 30% by weight isobutane. Thus, both feeds to the alkylation reactor are obtained from a Fischer-Tropsch reaction. Preferred techniques for reducing oxygenate levels include extractive or azeotropic distillation, decarboxylation, adsorption and water extraction or water washing. Alcohols recovered from the Fischer-Tropsch process can be dehydrated to provide additional C3-C4 olefins. Also, 1-butenes present in the olefin stream can be isomerized to 2-butenes to improve the octane number.